MEasure the size of the fans from mounting hole to mounting hole. Then go to eBay and buy a few fans of the same size, looking for dba readings in the lower 30's. You could buy them at your local Radio-Shack or PC dealer, but they'll be $19.95 each there. Very important: Also buy some fan guards! Otherwise you'll soon be making a trip back to buy fans with blades still on them. Don't ask me how I learned about this.
If the fans are going to be on for more than a few hours a day, be sure they're the $1 more expensive ones with BALL BEARINGS. The brass bearing ones will last about 18 months if they're on all the time.
Then swap the fans and see how that sounds. Probably a lot better. Also feel-up your disk drives after a few hours of use. If they feel just warm, the new fans have enough air flow. Otherwise if they're uncomfortably hot, you might need a bit more cooling.